Place the laptop, without the battery, in an oven with a very low heat of 110 to 120 f max. If you have to leave the door of the oven wedged partially open to keep the heat down, do it. Place an oven thermometer in with. Leave in for 12 hours. I would do it as soon as possible.
If only rainwater was involved, you have a good chance of resurection; but you must act quickly.